1. Field of the Invention
This invention relates to methods for altering the surface energy of a material. More specifically, the invention is a method of modifying the surface energy of a substrate material using laser ablation where the method does not require the use of any template, mask, coating, or post-ablation processing.
2. Description of the Related Art
Interfacial interactions are governed by the surface energy of the contacting materials. The proclivity for favorable (adhesive or wetting) or unfavorable (adhesive or non-wetting) interactions will depend on the relative magnitude of these surface energies. As a result, the ability to controllably alter a material's surface energy is of great significance. Differences in the surface energy of materials can often be observed via water contact angle values.
Surface chemistry and topography contribute to a material's surface energy. Chemical functionalities present on a material's surface alter surface energy through intermolecular forces (e.g., dispersion forces, dipole-dipole interactions, polarizability, etc.). Topographies impact surface energy by altering the contact line between two surfaces. For liquid droplets on a solid surface, this is referred to as the three-phase contact line. The continuity of the three-phase contact line dramatically influences the wetting behavior and surface energy of the solid substrate. For a rough surface or one with a high density of topographical features, the contact line can be continuous or discontinuous. If the contact line is continuous, the surface is said to exhibit wetting in the Wenzel state where the liquid has penetrated into the interstices of the topographical features. This results in a larger contact area between the liquid and the surface than would be observed on a flat surface of the same dimensions and often leads to adhesion promotion. For discontinuous contact lines, the liquid has not penetrated the surface topography interstices resulting in a reduced contact area relative to a flat surface and a Cassie-Baxter wetting state. Discontinuous contact lines often correlate with surfaces that exhibit adhesive interactions and superhydrophobicity, where water contact angles exceed 150°.
Surface preparation for adhesion promotion utilizes methods to increase the surface energy of a material either chemically or topographically. State-of-the art surface preparation for metal adherends typically involves grit-blasting followed by multiple chemical oxidative treatments and subsequent application of a primer or coupling agent. The use of chemical surface preparation techniques for metallic substrates requires large volumes of environmentally toxic materials. For the surface preparation of reinforced composite materials grit-blasting, manual abrasion, and peel ply treatments are commonly employed. All of these surface preparation techniques are not ideal in terms of adhesively bonded structures because of variations in their application (i.e., there can be dissimilarities across surfaces because of different operators, operator error, or other inconsistencies inherent with these techniques). Similarly, these techniques can alter the bonding interface due to debris and introduction of surface curvature. Thus, an environmentally benign, rapid, scalable, precise, and highly reproducible surface preparation technique for the purposes of adhesion promotion would mitigate many of these shortcomings and be of great utility.
Surface preparation for adhesion promotion requires a reduction in the surface energy of a material. Once again, this can be achieved both chemically and topographically. For a smooth surface, water contact angle values>120° cannot be achieved solely through surface chemical modification. For the generation of superhydrophobic surfaces, which are akin to low surface energy materials, topographical modification of the material is required. Superhydrophic surfaces are known to mitigate particulate adhesion, which not only changes the appearance of an exposed surface, but also can impair or reduce the efficacy of the impacted structure. Exterior building wall fouling as a result of particulate accumulation often results in acceleration of degradation due to the introduction of organic matter and a viable matrix for mold and fungal growth. Similarly, solar cell efficiency rapidly diminishes as a result of surface contamination by particulate adhesion. Frictional wear also increases considerably due to the presence of particulate matter. Therefore, identification of a method to reduce the propensity for particulate adhesion by lowering surface energy via topographical modification would be useful to a broad range of materials applications.
The current state-of-the-art surface treatment for aluminum metal bonding for most applications is phosphoric acid anodization, chromic acid anodization, or chromic acid etching. The preferred surface treatment method for both production and repair of titanium, stainless steel, and nickel substrates is a wet chemistry process called sol-gel. Although great progress has been made over the past few decades in improving the performance and durability of bonded metal structures, there remains much room for improvement. Furthermore, one of the greatest challenges facing the metal bonding industry today are the changing safety and environmental regulations that control the use of chemicals used to process bonded metal structures. There is a great need to minimize or eliminate the use of toxic chemicals and volatile organic solvents.
The current state-of-the-art for preparing composite surfaces for bonding uses abrasive techniques such as grit-blasting, surface roughening (manual abrasion), and peel plies. These methods lack precision and reproducibility thereby making quality control difficult. Surface preparation methods for composites are currently process controlled and no viable methods exist to assess whether a surface is adequately prepared. Also, the reliability of peel ply methods needs additional improvement from a reproducibility and contamination viewpoint. While grit-blasting of composite surfaces is widely used, the understanding of the effects of microcracking and grit embedment still need to be understood within the context of the durability of the bond as it ages. From an environmental and health perspective, the containment of the grit blast media and exposure of workers to grit dust are also issues.
A technique for generating patterned surface topographies that includes laser ablation was reported by Jin et al. in “Super-Hydrophobic PDMS Surface with Ultra-Low Adhesive Force,” Macromolecular Rapid Communications, 26, 1805-1809 (2005). However, the technique described therein requires laser ablation of a support substrate utilizing a mask or template to impart the desired surface pattern. The ablation process also results in deposition of ablated debris on the treated surface. As a result, this ablated material forms topographical features on the nanometer scale that render the material superhydrophobic. A further example of laser ablation patterning was reported by Schulz et al. in “Ultra Hydrophobic Wetting Behavior of Amorphous Carbon Film,” Surface and Coatings Technology, 200, 1123-1126 (2005). Surface energy reduction described therein requires the use of both a laser and an arc plasma generating device. Additionally, the surfaces were rendered superhydrophobic only after the addition of an undisclosed hydrophobic film.